This invention pertains in general to a method and means for accurately determining the flow rate of gas through a pipe. More specifically, the present invention describes a method and apparatus for determining to a high degree of accuracy the flow rate of high pressure natural gas through a pipeline by means of critical flow nozzles.
In the purchase and sale of natural gas, extremely large volumes of natural gas are continuously purchased and sold. Sales meters are installed in the gas lines to measure the volume of gas passed through the line and, thus, determine the amount of money which must be paid for the gas. Because tremendous volumes of gas are being purchased and sold, errors of only a few tenths of a percent in the meters can result in million of dollars in over or under payment over a long period of time.
In order to calibrate these sales meters, a "critical flow nozzle" can be used in series with the sales meter for a short period of time, i.e., a nozzle in which the gas reaches supersonic speeds in the nozzle. As is well known to one skilled in the art, the gas flow rate through a critical flow nozzle is dependent only on the pressure and temperature upstream of the nozzle and the thermodynamic properties of the gas. Using equations well known to one skilled in the art, the gas flow rate through a critical flow nozzle can, therefore, be easily determined. A gas meter in series with a critical flow nozzle can, therefore, be easily calibrated.
The equations for the flow rate through a critical flow nozzle for some gas mixtures are highly accurate. However, some gas mixtures do not have an accurate equation of state. At extremely low pressures, the ideal gas law is an adequate equation of state. Various equations are known which account for the increased molecular interactions occurring at higher pressures. At very high pressures, even the most sophisticated of these equations become somewhat questionable. Further, when a complex mixture of gases (such as natural gas) is being measured, these equations become increasingly unreliable. These problems are further complicated when highly polar molecules are present, such as CO.sub.2 and H.sub.2 S. As petroleum reserves are depleted, natural gases containing higher percentages of these components are being produced.